Many automobile manufacturers are offering four wheel steering on some or all of their vehicles. Four wheel steering offers improved performance in turning at both high and low speeds.
One characteristic of all steering systems, two and four wheel alike, is that they prescribe to Ackerman geometric constraints if they are to have error free turning. Error free turning occurs when all four wheels turn about a single point, the point about which the vehicle is turning. A steering system that implements error free turning is an Ackerman steering system.
All four wheels of a vehicle are turning about a single point when lines drawn through the center of each wheel, perpendicular to the plane of those wheels, all meet at a single point. That point is the point about which the vehicle is turning. In a two wheel steering system, the point about which the vehicle turns lies along the axis of the two wheels that do not turn.
Not all steering systems are error free, however. If the steering system of a vehicle does not prescribe to Ackerman geometry, the wheels of the vehicle will be turning about different points. In these situations, the wheels fight with each other to force the vehicle to turn about the point which each wheel is turning. As a result, one or more of the wheels end up being dragged in a direction slightly different from that in which it is steered. The affect on the tire is called tire scrub, and it adds to the wear of the tire.
Ackerman geometry has been implemented in two wheel steering systems for years. It can be implemented to close approximation with mechanical linkages. The advent of four wheel steering, however, has complicated the task of achieving Ackerman geometry. In a four wheel steering system, the point about which the vehicle turns for error free turning is no longer along the axis of the rear wheels. The rear wheels are no longer on the same axis and each rear wheel must be steered so that it is turning about the same point that the front wheels are turning about.
One problem with providing error free four wheel steering is that the rear wheels of a four wheel steering system often have a travel limit. In other words, the rear wheel can often turn only so far, for example four degrees. The travel limit may depend upon whether the wheel is on the inside of the turn or the outside of the turn.
Another problem with providing error free four wheel steering is that the rear wheels are steered in two modes. An example of a two mode system is a system in which the rear wheels turn out of phase (steered in the opposite direction that the front wheels are steered) when the vehicle is traveling at slow velocities and the rear wheels turn in phase (steered in the same direction that the front wheels are steered) when the vehicle is traveling at higher velocities. U.S. Pat. No. 4,796,904, issued to Kubo et al., discloses a mechanical linkage steering system for four wheel steering which approximates an Ackerman steering system, but only when the rear wheels are steered out of phase, and it is only effective at certain steering angles.